docs: def _compute_intra_event_std(self, C, vs30, pga1100, sigma_pga):
          """
          Returns the intra-event standard deviation at the site, as defined in
          equation 15, page 147
          """
          # Get intra-event standard deviation at the base of the site profile
          sig_lnyb = np.sqrt(C['s_lny'] ** 2. - C['s_lnAF'] ** 2.)
          sig_lnab = np.sqrt(sigma_pga ** 2. - C['s_lnAF'] ** 2.)
          # Get linearised relationship between f_site and ln PGA
          alpha = self._compute_intra_event_alpha(C, vs30, pga1100)
  
          return np.sqrt(
              (sig_lnyb ** 2.) +
              (C['s_lnAF'] ** 2.) +
              ((alpha ** 2.) * (sig_lnab ** 2.)) +
              (2.0 * alpha * C['rho'] * sig_lnyb * sig_lnab))